1. Field of the Invention
The present invention relates to a process for the preparation of compositions comprising vinyl aromatic copolymers prepared by the emulsion polymerisation process and having salt inclusions resulting from their preparation, which compositions are distinguished by an improved surface quality after storage under warm moist conditions and in that respect are suitable for the production of moulded bodies having an age-resistant defect-free class A surface.
The present invention relates in addition to the compositions prepared by the process according to the invention and to their use in the production of moulded bodies with a class-A-surface requirement and a partial or full high-gloss finish, which can optionally be subjected partially or completely to a further surface treatment step by, for example, painting, in-mould decoration, metallisation by vacuum deposition or galvanisation.
2. Description of Related Art
Compositions comprising vinyl aromatic copolymers prepared by the emulsion polymerisation process and having salt inclusions resulting from their preparation are known in the literature. Sources of such salt inclusions resulting from the preparation are many and varied, for example emulsifier solutions, polymerisation initiator solutions, buffer solutions and precipitating agent solutions which are used as auxiliary substances in the emulsion polymerisation process and which, depending on the process, remain in the material or are removed from the material again only incompletely upon working up of the polymer.
In particular, the precipitation of vinyl polymer latices prepared by emulsion polymerisation by means of the addition of acids and/or salts, which is generally carried out in traditional processes, such as, for example, in EP 459 161 B1, DE 2 021 398 and DE 28 15 098, contributes to a considerable extent to the salt content of the final polymer, because such salts can generally be removed from the product again by subsequent process steps (washing) only unsatisfactorily or with a high outlay (energy and water/waste water). There are used as coagulants, for example and preferably, aqueous solutions of water-soluble salts such as, for example, alkali, alkaline earth or aluminium chlorides, sulfates, nitrates, phosphates, acetates, formates, aluminates or carbonates, particularly preferably aluminium chloride, calcium chloride and magnesium sulfate solutions, optionally in combination with inorganic or organic acids such as, for example, hydrochloric acid, sulfuric acid, phosphoric acid, boric acid, formic acid, acetic acid, propionic acid and citric acid.
It is described in the literature that such salt inclusions in compositions comprising vinyl aromatic copolymers can lead to undesirable effects.
WO 2009/071537, for example, discloses that magnesium compounds and/or calcium compounds in impact-modified vinyl aromatic copolymers selected from the group of the acrylonitrile-butadiene-styrene copolymers (ABS), acrylonitrile-styrene-acrylate copolymers (ASA) and methacrylate-acrylonitrile-butadiene-styrene copolymers (MABS), optionally comprising polycarbonate and additives, lead to undesirable film formation on the moulding tool during thermoplastic shaping by means of injection moulding or extrusion, and claims in that respect such compositions having a content of magnesium compounds and/or calcium compounds of from 0 mg/kg to 100 mg/kg. The emulsion polymers used in those compositions are not precipitated by the addition of magnesium sulfate solution, as is traditionally conventional, but by freeze precipitation on a flake ice machine.
WO 98/28344 discloses a process for the continuous coagulation of aqueous dispersions of graft rubbers by means of shear, which process overcomes the known disadvantage of precipitation by means of acids and/or salts as coagulants that impurities often remain in the worked-up polymers which can lead to an impairment of the product properties.
A problem of thermoplastic compositions comprising vinyl aromatic copolymers prepared by the emulsion polymerisation process and having salt inclusions resulting from their preparation is that, when mouldings produced therefrom are exposed to moisture (for example condensation or atmospheric moisture), in particular at elevated temperatures, they have a tendency to form undesirable surface defects (blister formation), which restrict the use of such compositions in mouldings having a high-gloss finish and a class A surface requirement.
EP 2 398 842 A1 discloses a compounding process for the preparation of impact-modified polycarbonate compositions having a reduced content of volatile organic compounds, in which there are added to the pulverulent graft polymer used as impact modifier from 2 to 40 wt. %, based on the sum of impact modifier and water, of liquid water, and the premixture so prepared is used in the compounding of the impact-modified polycarbonate compositions. This process corresponds to a comparative example in this application.